Thread: realoc, calloc

Originally Posted by brewbuck

But assuming that freshly-allocated memory will contain all zeros is not correct (I'm not saying that's what you're saying, matsp)

I agree with that - first of all, there's no way to KNOW if the memory is "fresh" or "old and mouldy" when you ask for memory from malloc - or when you ask from the OS directly. It may use memory previously owned by this process or some such.

The only way to "know" what's in memory recently allocated is if you use calloc or you fill it yourself. Anything else is "undefined".

--
Mats

Debug versions of malloc specifically set the memory to known "magic" numbers to assist with such things as
- use before initialisation
- use after free.

If you see these magic numbers in the memory you allocated, you know you've messed up.

But other than that, definitely assume it's junk to begin with.

calloc is a waste of effort if the very next thing you're going to do is say call strcpy() or fgets() which will simply overwrite what is already there with a valid string.

Most of the time, you'll be wanting to write the data you really want to the memory first, so just call malloc. On some occasions, having zeroed out memory is nice since it saves you a memset call.

But I would regard blanket use of calloc by a programmer as showing signs of not knowing what they were doing. Especially where it is immediately obvious that the "zeroising" aspect was a complete waste of time.

Even when calloc would have worked, I often end up with malloc + memset.

Being over-reliant on it can be a trap, since there is no 'calloc' version of realloc(), which will always leave you with uninitialised memory in the extra space.

Originally Posted by eXeCuTeR

So you're saying that calloc is better than malloc and I should use calloc?

Is a screwdriver better than a spanner? No. You should use the RIGHT tool for the job. And whilst you can use a screwdriver (or spanner) to hammer in nails, a hammer is actually much better for it.

What makes it hard is that there is no firm and fast rule. But if you don't immediately fill your malloc'd memory with something, then you probably should use calloc(). There's obviously nothing wrong with always using calloc() - it is better than getting it wrong the other way. But it will cause un-necessary memory writes, when you fill it with zeros only to immediately stuff something else on top of those zeros.

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Mats

You've really gotta start looking at what all these functions do in your manual pages.
We're certainly not going to explain the entire standard C library to you one message at a time.

You only do strcpy() if you want to copy a character string.

You use fgets() to read a string from the console or a file.

memset is a function that fills a set amount of memory with a the value of a byte - typically zero, but other values can be used.

--
Mats

Originally Posted by eXeCuTeR

Could you atleast explain me why strcpy(p, str)? why do I need to copy the allocated memory into str?

Ehm, it's the other way around. strcpy(destination, source) - so copies from the second argumen TO the first one.

--
Mats

Originally Posted by eXeCuTeR

So why do I need to put str in p? what different does it make whether the string/w/e is on str or p?
You answered it yesterday I guess, but I forgot ^^

hmm..
sometimes if you want to change the string you dont have to go to every point and change it..
if you have something like this

Code:

main()
{
    char str[]="I am string1";
    printf("%s\n",str);
    
    //now lets play with string in some function
    strcpy(str,"I am string2");
    
    printf("%s\n",str);
}
/**************************************************************
else the basic way will be
str[0]= 'I';
str[1]=' ';
str[2]='a';
str[3]='m';
...
str[13]='2';
all these lines of code can be done in one single call strcpy(str,"I am string2")
***************************************************************/

Originally Posted by eXeCuTeR

Finally got it
2. Why do I have (note the comment):

Code:

int *p, str;
p = malloc(sizeof(int));
strcpy(p, str); // Why do I have to do that? Salem mentioned that.
// He also mentioned fgets - how do I do it with fgets?

That last line of code is not right in many ways.
First, you cannot sensibly use strcpy on integer pointers.
Secondly, you've declared str as simply an int so it wont compile.
Hopefully you would never use malloc for something of such a small fixed size either.